In the printing industry it is sometimes necessary to retain a plate on the cylindrical surface of a vacuum drum. For example, many computer-to-plate or computer-to-press systems image a lithographic printing plate which is held onto the outside surface of a rotating drum. In such systems the rotating drum has a cylindrical surface onto which a plate can be held. The cylindrical surface is typically perforated with an array of holes or other apertures. Air pressure in a plenum behind the cylindrical surface is reduced by a suitable vacuum pump. The resulting pressure differential between atmospheric pressure on one side of the plate and a reduced pressure on the side of the plate in contact with the cylindrical surface holds the plate against the cylindrical surface.
While such vacuum drums are well adapted to holding flexible films, they cannot always reliably hold metal printing plates. Metal printing plates have greater stiffness and thickness than films. Because of this, metal plates tend not to seal to the cylindrical surface of a vacuum drum as well as films. This reduces the pressure differential across the plates and reduces the forces which hold the plates to the drum. Leakage is especially significant in the areas near the edges of the plates.
Because metal plates are often heavier than comparable films the centrifugal forces which act on such plates when they are mounted on a rotating drum tend to be larger than those which act on films. For heavy plates mounted on a large drum which is rotating with a high angular velocity the centrifugal forces acting to pull the plates off of the rotating drum can approach the forces caused by the pressure differential across the plate which tends to hold the plate to the drum. The pressure differential across the plate is no more than one atmosphere and tends to be less in regions adjacent the edges of the plate where leakage is a problem.
Prior art approaches to the problem of holding plates to a rotating vacuum drum provide mechanical or magnetic clamps to hold both edges of a plate to a rotating drum. Providing such clamps is complicated because it should be possible to easily and quickly affix plates of different lengths to the same drum. A first clamp for holding a first edge of a plate may be at a fixed location on the surface of a drum. A second clamp for holding a second edge of the plate must be movable relative to the first clamp.
It is known to use magnetic clamps for the second clamp. A magnetic clamp consists essentially of an elongated member which contains one or more magnets. The magnets are attracted to the drum. The magnetic clamp may be simply placed over a second edge of a plate to hold the second edge of the plate to the drum. Such magnetic clamps have the advantage that they may be positioned as necessary to hold down the second edge of a plate. A disadvantage of such magnetic clamps is that the magnets which are required in the clamp are reasonably heavy. As a magnetic clamp is moved to accommodate plates of different lengths the balance of the drum will change. This can cause undesirable vibrations, especially when the drum is rotated at higher angular velocities. Furthermore, if a heavy magnetic clamp does fly off a drum during use it can cause significant damage to adjacent parts of the machinery.
It is known to hold steel plates to a drum by providing magnets inside the drum. This approach does not work with aluminum plates.
There is a need for a clamping system suitable for clamping the edges of printing plates of different sizes to a vacuum drum which avoids or reduces the above-noted disadvantages of currently available clamping systems. There is a particular need for such clamping systems that may be used on existing vacuum drums without the need to make structural modifications to the existing vacuum drums.